A buffered hydrofluoric acid solution (this may be referred to below as a BHF solution) is used, for example, in a semiconductor device etching process. Because the speed of this etching is determined by the concentration of the HF which is the principal constituent in the BHF solution, conventionally, as is shown, for example, in Patent document 1, the HF concentration is measured and controlled using an optical measurement device.
In recent times, in conjunction with the increasing miniaturization of semiconductor devices, an extremely low concentration BHF solution is sometimes used in which the HF concentration has been diluted to one tenth or one hundredth of the conventional concentration.
However, in the above-described conventional optical measurement device, it is difficult to accurately measure the HF concentration in a BHF solution in such a low concentration region.